What I can infer from here, which is quite obvious, is that client sent some data while the connection was supposed to be closed.
Could this be a case where I initiated an ssh session, got had a Man-in-the-Middle soemwhere without me knowing, and after I closed the SSH session he tried to connect again?

2 Answers
2

This can happen in cases where one side does an RST not a proper close, and if there's enough latency data can be in-flight for a while afterwards, or even due to queueing after both sides have closed.

In short, data after close is fairly innocuous as the session had to be up in the first place to cause that, so don't worry.